Friction between moving piston assembly parts and cylinder bore accounts for the largest part of mechanical energy losses in internal combustion engines. Friction also leads to piston ring wear which affects compression sealing and oil consumption. Therefore, there is a general request to provide cylinder bore surfaces having as low friction and experiencing as little wear as possible, while maintaining their optimal tribological properties from the first day in exploitation and for entire engine life span.
Prior-art approaches to provide low-friction surfaces comprise use of PVD and CVD coatings, plasma-sputtering, solid lubricants films and polymer-bonded solid lubricant coatings. Thus, the published US patent application 2005/0214540 describes PVD/CVD coatings for pistons, and the U.S. Pat. No. 4,629,547 describes low-friction boron-containing films obtained by plasma sputtering. The utility of certain solid film lubricants has been known for quite some time. Here below are just a few examples presented. The U.S. Pat. No. 1,654,509 describes the use of graphite embedded into a metal binder to make an antiwear coating for bearings. The published British patent application GB 776502 A describes protective films formed by treatment with vaporized reactive substances containing phosphorus, sulfur, selenium or halogen atoms. GB782263 shows that sulfurization of ferrous metal parts by heating the parts to a temperature above 500° C. in a fused salt bath containing alkali metal cyanide, alkali metal cyanate and active sulfur improves resistance to wear and seizure. The published international patent application WO03/091479A describes chemical treatment for piston rings and pistons by heating in oil containing appropriate additives. The U.S. Pat. No. 5,363,821 discloses use of graphite, MoS2, BN solid lubricants incorporated into a polymeric carrier/binder for making antifriction coatings at the cylinder bore walls by spray-application with subsequent thermal fixation. The Japanese patent application 2004-76914 discloses a method for production of a low friction coating by encapsulation of molybdenum and sulfur into a polyamideimide resin matrix.
Common for most solid lubricant systems is that the lubricant is deposited onto the surface either as a pure lubricant substance or as a lubricant in a bearer substance. The deposition can be followed by different kinds of post treatments, typically thermal treatments. The lubricants will thus be provided as a layer on top of the surface to be lubricated.
A manufacturing method to produce low-friction surfaces by using a mechanochemical process, conditioning by means of tribochemical reactions, has been described in the published US patent application 2013/0104357 A1 or the published US patent application 2010/0272931 A1. The method involves rubbing a hard tool against the component surface while applying a sufficiently high load in the presence of a process fluid containing refractory metal dichalcogenides solid lubricant precursors. Conditioning by means of tribochemical reactions has been shown to lead to significant improvement in terms of surface roughness, wear resistance and friction reduction. In contrary to other previous solid lubricant systems, the so produced surface composition is created as a modification of the original surface and becomes thus an integrated part of the originally provided surface.
The Conditioning treatment by means of tribochemical reactions can be viewed as an in-manufacture running-in process. Running-in, or breaking-in, of an engine smoothes down surface irregularities and reduces localized pressure between various rubbing parts; the ring/bore system and valve train, especially for flat-tappet cammed engines, being the primary points of concern. Whereas engine running-in is a well-established procedure for training new or rebuilt engines in order to maximize their power output and durability, it has never been attempted to carry it out at a component level—as a dedicated finishing operation during the component manufacture. Doing so allows one to optimize processing conditions for each component individually, thus maximizing the effect of the treatment.
This new type of surface treatment was initially performed using standard honing tools that have been equipped with working stones with very hard surfaces. Examples of standard honing tools can be found e.g. in the U.S. Pat. Nos. 1,955,362 and 2,004,949. However, since conditioning by means of tribochemical reactions, in contrast to traditional honing, is a non-abrasive method, operation based on prior art honing equipment with honing stones replaced by hard surface working stones was found to be far from ideal. It was for instance found that tool preparation took unreasonably long time, tool service life was far too short, process stability was poor, and the outcome of the treatment could vary from one setup to another.